This invention relates to a ceramic thermal barrier coating on a substrate, and, more particularly, to the in-situ formation of a thermal protective coating on the substrate.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the hot-section components of the engine. These components include the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases directly impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F. These components are also subject to damage by oxidation and corrosive agents, as well as impact damage and erosion by particles entrained in the combustion gas stream.
Many approaches have been used to increase the operating temperature limits and service lives of the turbine blades and vanes to their current levels, while achieving acceptable oxidation, corrosion, erosion, and impact resistance. The composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example by providing the component with internal cooling passages through which cooling air is flowed.
In another approach used to protect the hot-section components, a portion of the surfaces of the airfoil sections of the turbine blades and/or vanes is coated with a thermal barrier coating system. The thermal barrier coating systems typically include a bond coat that contacts the substrate, and a ceramic thermal barrier coating (TBC) layer overlying the bond coat. The bond coat protects the articles against the oxidative and corrosive effects of the combustion gas. The ceramic layer provides thermal insulation and some environmental protection. The turbine blades and turbine vanes are thereby able to run cooler and are more resistant to environmental attack in the presence of the thermal barrier coating systems.
Although the thermal barrier coating approach is operable and widely used, there is opportunity for improvement. In the existing approach, if the thermal barrier coating is significantly damaged during service such as by an impact that chips away a region of the ceramic layer, the thermal insulation is lost. The loss of the thermal insulation properties may lead to catastrophic damage to the substrate. There is a need to make the protection of the substrate less susceptible to such damage of the thermal barrier coating. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for protecting a substrate using a ceramic coating which is comparable in some ways to a thermal barrier coating. The present ceramic coating differs from the conventional thermal barrier coating in several ways. Its physical structure provides for a ceramic-based coating, but utilizes a trappedgas space to reduce thermal conductivity. It is more resistant to damage such as impact damage than a conventional thermal barrier coating. The present ceramic coating forms a ceramic thermal barrier in an in-situ fashion. As a result, the ceramic coating is selfforming during an initial break-in period and/or during service. It forms selectively where needed. It is also self-healing during service in the event that it is damaged. A method for protecting a substrate comprises the steps of providing a substrate and applying a ceramic coating overlying and bonded to the substrate. The ceramic coating comprises an open-cell solid foam of ceramic cell walls having an interconnected intracellular volume therebetween which is filled at least in part with a metallic alloy. The ceramic coating has an exposed surface remote from the substrate. The method further includes heating the exposed surface of the ceramic coating to an exposure temperature such that at least some of the metallic alloy is lost from the intracellular volume. The heating to the exposure temperature may occur during manufacturing or an initial break-in period, or it may occur during service.
The exposure temperature is typically at or slightly above the solidus temperature of the metallic alloy. The metallic alloy at least partially melts, and is lost from the exposed surface of the ceramic coating by being blown out by the gas flow or flung outwardly by centrifugal force. Any other operable way of removing the metallic alloy at the exposed surface may also be used. When the metallic alloy is lost, there remains the ceramic cell walls with empty porosity therebetween. The empty porosity has two beneficial effects. It reduces the thermal conductivity of the ceramic coating by providing an insulating gas barrier within the intracellular volume. The empty porosity also reduces the weight of the coated substrate, thereby reducing loading on the supporting structure.
The desirable structure forms only as needed, in the regions that are exposed to the most severe heating and other environmental conditions. In other regions that are less severely exposed and/or underlie the ceramic cell walls with intracellular porosity, the intracellular metallic alloy remains. The intracellular metal imparts improved resistance to brittle fracture and thence reduced susceptibility to impact damage in those other regions.
This mode of formation of the insulating structure also renders the ceramic coating self-healing and self-repairing. If a piece of the ceramic coating is worn or chipped away or otherwise removed during service, fresh ceramic cell walls and intracellular metallic alloy are exposed to the environment. The metallic alloy is removed by the same mechanisms as discussed earlier, leaving new solid-foam ceramic material with intracellular porosity to protect the underlying substrate.
The ceramic coating is preferably formed by depositing a precursor material onto the surface of the substrate, either with or without a bond coat. The precursor material comprises a sacrificial ceramic, and a reactive metal which is reactive with the sacrificial ceramic to form an open-celled ceramic foam. The sacrificial ceramic and the reactive metal are reacted to form a ceramic coating having an exposed surface and ceramic cell walls of an oxidized ceramic of the reactive metal, and an interconnected intracellular volume therebetween filled at least in part with a metallic alloy. The metallic alloy may be replaced with another metallic alloy if desired. In a typical case, the sacrificial ceramic is silicon dioxide (silica) and the reactive metal is aluminum. The resulting ceramic cell walls are aluminum oxide (alumina) and desirably constitute from about 60 to about 80 percent by volume of the ceramic coating. The initial intracellular metallic alloy is an aluminum alloy, and it may be replaced at least in part with a nickel alloy such as a nickel-base superalloy.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.